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The British Wind Energy Association, which promotes the UK windfarm industry, has been forced to halve its figures on carbon-emission reductions by the Advertising Standards Authority (ASA).

The BWEA had formerly made its calculations on the basis that every kilowatt-hour (or "unit") of electricity generated by a wind turbine would mean 860 grams of CO2 not emitted by fossil-fuelled power stations. Now, however, it has cut that claim to 430 grams per kWh, following a landmark ASA ruling last year against RWE nPower.

"The 860 gCO2/kWh figure was hard to validate," ASA spokesman Matt Wilson told the Reg today. "Following the ruling there was a consensus that the figure be lowered."

Setting a new figure was difficult, however, as judging just how much carbon is saved per kWH of wind energy generated is almost impossible to do with any accuracy. The amount of carbon generated by fossil stations per kWh varies according to the technology used and the age of the facility, and different stations are on line at different times.

"It was a fiendishly complex process," said Wilson. "In the end, we're not experts in this area. We can say a figure is misleading, but we can't say what the true value is."

The BWEA has now recalculated its carbon savings figures based on 430 gCO2/kWh, which will effectively mean that the amount of wind turbines required to achieve a given level of carbon savings has doubled. This is an acknowledgement of the fact that some older and dirtier fossil stations - particularly coal ones - have closed, and the increasing prevalence of efficient combined-cycle machinery in the gas sector.

At present, the BWEA still makes no allowance in its calculations for the carbon effects of uncontrolled variability in wind supply, saying "this is unlikely to become a significant issue until wind generates over 20 per cent of total electricity supply".

A report written for the Renewable Energy Foundation in the summer said that a substantial wind base would involve more carbon burden than current figures suggest. This was owing to the need for backup gas turbine power during calms - and the fact that irregularly-run turbines would be dirtier than ones run on a predictable schedule.

The BWEA is also sticking to its line that a "normal" 2 megawatt turbine "produces enough electricity each year to meet the needs of 1,000 homes". This is true - provided that those homes are well supplied with gas or heating oil to turn into carbon emissions.

In a post-fossil future where heating, cooking and hot water were all electric - and assuming no rise in domestic energy use overall - such a turbine would actually meet the needs of 214 homes. Provided there was also a backup fossil power station and/or pumped storage hydropower reservoirs, of course.

As of publication, the BWEA still hadn't responded to requests for comment. ®